In recent years, attempts to selectively recover gases, particularly carbon dioxide gas, have been made using a polymer membrane. Carbon dioxide gas is one of the causes of global warming, so it is preferable to selectively recover carbon dioxide from the exhaust gas from power plants and factories. Examples of a method for separating and recovering carbon dioxide from the exhaust gas include a method of absorbing carbon dioxide into a solution, a method of adsorbing carbon dioxide with an adsorbent, a method of separating and recovering with a membrane having high selective permeability to carbon dioxide, and the like. Particularly, the membrane separation method is preferable because energy and cost required for separating carbon dioxide can be kept low.
Development of polymer membranes with high selective permeability to specific gases is vigorous. For example, Patent Document 1 discloses that a polymer membrane made of a crosslinked product of a mixture of polydimethylsiloxane and polydiphenylsiloxane can be used as an oxygen gas separation membrane having high oxygen permeability and excellent heat resistance and mechanical properties. In addition, Patent Document 2 discloses that a transition metal salt-polymer mixed membrane composed of a transition metal salt and a polymer not having a functional group forming a complex with the transition metal salt has high permeability and selectivity to alkene hydrocarbons such as ethylene. Patent Document 3 discloses that a gas separation membrane including a separation-active membrane containing a crosslinked polymer and a compound having a molecular weight of 150,000 or less capable of interacting with an acidic gas and a porous membrane having a molecular weight cutoff of 500,000 or less is suitable for separation of acidic gas such as carbon dioxide. In addition, Patent Document 4 discloses a membrane in which an organometallic complex in which an aryl group and a heterocyclic group are coordinated to two molecules of copper (II) atoms or rhodium (II) atoms is dispersed in a resin as a membrane for separating an organic compound such as a lower alcohol from a solution.